The present invention relates to a homogeneous charge compression ignition engine and a method for operating a homogeneous charge compression ignition engine. More particularly, the present invention relates to a homogeneous charge compression ignition engine optimal for use as a stationary engine for a private power generator or the like.
Conventional engines may be categorized into two types, a spark ignition (SI) engine and a diesel engine. The thermal efficiency of the SI engine may be increased by causing the air-fuel mixture to be lean. However, there is a limit to the concentration ratio at which a spark may propagate. Thus, an SI engine requires adjustment of the amount of air with a throttle valve. As a result, the thermal efficiency of the SI engine is inferior to that of a diesel engine. Conversely, a diesel engine has satisfactory thermal efficiency. However, the diesel engine does not sufficiently mix fuel and air. As a result, NOx tends to be generated due to local combustion of fuel at high temperatures, and soot tends to be generated due to local enrichment.
In comparison with such engines, a homogeneous charge compression ignition engine premixes air and fuel. Thus, the possibility of local high temperature combustion or enrichment is small, and the generated amount of NOx and soot is subtle. Further, in a homogeneous charge compression engine, chemical changes cause ignition. Thus, the dependency on the concentration ratio is lower than that of an SI engine. As a result, the homogeneous charge compression ignition engine is capable of causing air-fuel mixture to be significantly lean, while achieving thermal efficiency at the same level as a diesel engine. With such advantages, homogeneous charge compression engines are receiving much attention. However, in a homogenous charge compression ignition engine, excessive heat would result in sudden combustion, and insufficient heat would result in misfires. Thus, in comparison to other engines, misfires, knocking, and pre-ignition are apt to occurring more easily. This tends to narrow the operable range of the homogeneous charge compression ignition engine.
A low NOx emission four-cycle engine that reduces the amount of hydrocarbon (HC) emitted together with exhaust gas by taking advantage of homogeneous charge compression ignition has been proposed (refer to, for example, Japanese Laid-Open Patent Publication No. 2000-64863). The engine described in the publication includes a variable valve actuation mechanism that varies the valve timing of an intake valve and an exhaust valve in accordance with whether the load of the engine is low or high. When the engine load is high, the valve timing is set so that the exhaust valve closes when the piston is near the top dead center. When the engine load is low, as the load decreases the valve timing is set so that the exhaust valve closes at an earlier timing before the piston reaches top dead center during the exhaust stroke. Further, when the engine load is in a high state, an igniter, which is arranged in the combustion chamber, ignites and burns fuel when the piston is near the compression top dead center. When the engine load is low, instead of igniting fuel with the igniter, the engine performs homogeneous charge compression ignition. That is, during homogeneous charge compression ignition, the variable valve actuation mechanism adjusts the timing at which the exhaust valve closes to perform internal exhaust gas recirculation (EGR).
A homogeneous charge compression ignition engine that re-circulates some of the exhaust gas and charges the re-circulated exhaust gas into a premixed mixture so that ignition easily occurs has also been proposed (refer to, for example, Japanese Laid-Open Patent Publication No. 2000-240513). The publication proposes the employment of a heat exchanger that heats fuel gas, intake air, or the air-fuel mixture. The heat exchanger heats the premixed mixture so that ignition easily occurs in the engine. Hot water generated by a water heater or the exhaust gas of the engine are given as examples of the heat source for the heat exchanger.
Further, a homogeneous charge compression ignition engine including a supercharger for supplying the combustion chamber with supercharged air (intake air), a cooling means for cooling with a coolant the air supplied and heated by the supercharger, and a supercharging detection means for detecting the supercharging state of the supercharger has been proposed (refer to, for example, Japanese Laid-Open Patent Publication No. 2001-221075). The supercharger is driven by the exhaust gas of the homogeneous charge compression ignition engine. A cooling tower air-cools the coolant. The homogeneous charge compression ignition engine also includes a concentration ratio setting means, which sets the amount of supplied fuel to set the concentration ratio of the premixed mixture, and an air temperature setting means, which sets the temperature of the air supplied to the combustion chamber. The engine stores the engine output in relation with the concentration ratio and the temperature of the supplied air under a supercharged state. The engine further includes an output setting means that sets the engine output by operating the two above-mentioned setting means based on the stored relationship to adjust the concentration ratio and the temperature of the supplied air.
The engine described in Japanese Laid-Open Patent Publication No. 2000-64863 performs internal EGR to increase the temperature of the premixed mixture and facilitate ignition. However, when the engine is idling, the thermal energy for heating the premixed mixture is insufficient when only internal EGR is employed. In such a case, homogeneous charge compression ignition becomes unstable.
The method described in Japanese Laid-Open Patent Publication No. 2000-240513 that re-circulates some of the exhaust gas to the intake passage and charges the exhaust gas into the premixed mixture to heat the premixed mixture, that is, the method that employs external EGR to increase the temperature of the premixed mixture has a low heating effect. This is because the high efficiency of homogeneous charge compression ignition results in a low exhaust temperature. Thus, the employment of exhaust gas recirculation (external EGR) slightly increases the temperature of the intake air. However, the specific heat of the air-fuel mixture is also increased. This may decrease the temperature at the compression end of the piston (top dead center), which affects ignition.
Another method described in Japanese Laid-Open Patent Publication No. 2000-240513 heats the premixed mixture with a heat exchanger using exhaust gas or hot water as a heat source. In this method, it may be difficult to heat the premixed mixture to a sufficient temperature due to the low exhaust gas temperature and the length of the passage extending through the heat exchanger. The publication describes a burner used for heating to obtain the hot water. When taking into consideration the fuel consumption of the burner, it is difficult to increase the energy efficiency of the entire apparatus even though homogeneous charge compression ignition ensures high combustion efficiency. Further, to heat the air-fuel mixture to a temperature that stabilizes homogeneous charge compression ignition just with the intake air, the intake air temperature must be increased to 120° C. or greater. Thus, when using hot water, measures, such as pressurizing the hot water, must be taken so that the hot water does not boil.
As described in Japanese Laid-Open Patent Publication No. 2001-221075, the homogeneous charge compression ignition engine includes a means for heating the air-fuel mixture with a supercharger that supplies supercharged air (intake air). This obtains the necessary heat when the engine is operating at a high speed. However, when the engine is operating at a low speed, the heating of the premixed mixture becomes insufficient. If another drive source, such as a motor, is used to drive the supercharger, the supercharger would be able to supercharge air. However, this would consume energy. Further, the employment of a supercharger would result in a complicated temperature adjustment means for the intake air. For example, a cooling means would become necessary.